The invention relates to a circuit for connecting a load to the high side of a DC power supply.
In many instances -- e.g. in the case of switching network components, motor controls (half and full bridge), in motor vehicles or in control technology -- a load must be connected to the positive side of the voltage supply. If an n-channel MOSFET is employed, it is necessary to use a gate voltage about 10 V above line voltage in order to attain full control.
A drive circuit for this purpose is discussed in German Offenlegungsschrift 32 09 070. There, a voltage step-up oscillator, provided with windings, derives a control voltage from the operating voltage. The control voltage is thereupon connected through by a microprocessor-controlled preamplifier stage. A disadvantage of this circuit is, however, that an auxiliary voltage must be generated. This in turn requires a transformer which contains coils and a core. For a number of reasons (limited integration capability, difficult mounting, substantial space requirement and high cost), this transformer should be eliminated if possible. Coilless "bootstrap" voltage step-up circuits are well known but they provide only for a dynamic circuit drive. P-channel MOS transistor technology would provide an especially simple circuit, but this approach is too expensive.
One object of the invention is to provide a circuit of this type that requires no additional external auxiliary voltage source, needs no inductors, and can be used in an integrated circuit.
Another object is, in general, to improve on known devices of this type.